Sweet, thanks
looks like I may have to invest in an ESR meter
You will have to study that circuit. These plate amps are among the most difficult devices to service and many experienced techs refuse to look at them. To be frank, they are the worst devices in the audio chain and the most unreliable.
If you are lucky you might have a blown cap.
One of the big problems is the digital switching power supplies which are a nightmare.
The big issue is your time out problem which is either the source of, or will lead you to the problem. The spec. is for that time out circuit to shut down after 15 min no signal. It seems yours has been way out of spec for a while.
Obviously the power supply is always on when plugged in. IC 1R2111 is a half bridge op amp chip, that receives the signal. Q 17, Q19, Q 21, and Q 23 form a transistor time out circuit. This is common in these devices but not the way I would do it. A CMOS logic chip and an LM series timer chip would be much better. It is most likely in that area where the trouble will be or lead you to it.
The other issue is that the manual calls for floating ground test procedures for some measurements. If you don't adhere to this you will do a lot more damage with your meter or scope and you may receive a serious personal injury. The best way to handle this, and this is what I do, is to have the device under test connected to a variable voltage transformer. I use a the time honored Variac. This makes service much safer and lessens the chance of damage.
This unit is not worth a lot of time. If there is nothing easy and obvious, I would buy and external amp and drive it with that. So in other words convert it to a passive sub.
